Aluminum foil (Al) is used extensively in the food packaging industry as a moisture and gas barrier. Because of foil's poor flex crack resistance, inability to form a hermetic seal and cost, it is typically combined with other materials such as paper, polyethylene (PE), oriented polypropylene (OPP) and polyesters such as polyethylene terephthalate (PET) in multilayer structures. Thus, multilayer structures comprising foil layers adhered to thermoplastic compositions are well known in the art of packaging films. Applications include many dry food packages such as powdered drink mix pouches as well as non-packaging applications. Some common structures include: Paper/LDPE/Al/LDPE; OPP/primer/LDPE/Al/LDPE; PET/primer/LDPE/Al/LDPE; and PET/print/primer/LDPE/Al/LDPE.
Among other functions, the paper, OPP or PET provide stiffness and a surface for printing. The primer allows low-density polyethylene (LDPE) to bond to the OPP or PET substrate; the internal LDPE layer provides adhesion to the aluminum foil and the external LDPE layer acts as a sealing layer.
These structures can be made by extrusion coating or lamination, which involves laying down a molten curtain of the polymer between the substrates moving at high speeds as they come into contact with a cold roll. Adhesion of polyethylene to foil can be accomplished by processing at high coating temperatures (300 to 330° C.) so that a portion of the polyethylene oxidizes. Oxidization of the LDPE creates polar species that provide moderate adhesion to the aluminum foil.
In many applications, rather than using a coated layer of LDPE as the sealant, a LDPE or LLDPE film is used. For example: Paper/LDPE/Al/LDPE/PE-film. The PE-film may be LDPE or LLDPE.
A problem with using LDPE to extrusion laminate PE-films to aluminum foil is that the adhesion of the LDPE to aluminum foil “ages down” with time. Adhesion of LDPE to aluminum foil is only marginally adequate to begin with, but over a time period of one to several weeks, the bond strength often declines to a level that is no longer functional for the application. One explanation is that the aging is associated with secondary crystallinity of the LDPE. During the lamination process, the LDPE is quenched very quickly and little primary crystallization can occur. Over time, small “secondary” crystals may form. As PE crystallizes, it shrinks. Shrinkage can put a stress on bonds and reduce peel strength. The LDPE-Al bond is the weakest of the bonds in the structure (bonds of the LDPE to the PE-film are typically inseparable).
In adhering foil to non-polar polymer compositions such as polyethylene, use of an additional polymeric composition as an adhesive or “tie” layer can be beneficial. However, polar polymers that adhere well to foil may not adhere well to non-polar polymers. Therefore, a tie layer must provide a balance of properties that allow it to adhere both to foil and to non-polar polymers.
Chemical primers are sometimes used to promote adhesion to thermoplastic substrates, but this adds costs and causes environmental concerns with solvent-based systems. Thus, it is desirable to develop a composition that adheres substrates like PET without the use of primers. An example is disclosed in U.S. Patent Application 2004/0001960.